How To Test The Type Of A Thrown Exception In Jest

Testing the type of a thrown exception in Jest can be crucial in ensuring your code behaves as expected and handles errors appropriately. Jest, the popular testing framework for JavaScript, provides a simple and effective way to test for specific types of exceptions thrown by your code.

Here's a step-by-step guide on how to test the type of a thrown exception in Jest:

1. **Writing your test case:**
First, you need to create a test case for the code that may throw an exception. You can use the Jest `expect` function along with a test function that throws an error of a specific type.

Javascript

function myFunction() {
       throw new TypeError('This is a type error');
   }

   test('testing exception type', () =&gt; {
       expect(() =&gt; myFunction()).toThrow(TypeError);
   });

In this example, we have a function `myFunction` that throws a `TypeError`. The test case uses `toThrow` matcher from Jest to check if the error thrown is of the `TypeError` type.

2. **Expecting specific error messages:**
Jest allows you not only to check the type of the thrown error but also the error message that comes with it. You can enhance your test case to verify the specific error message along with the type.

Javascript

function fetchData() {
       throw new Error('Failed to fetch data');
   }

   test('testing exception type and message', () =&gt; {
       expect(() =&gt; fetchData()).toThrowError('Failed to fetch data');
   });

Here, the `toThrowError` matcher is used to check if the error message matches the expected message.

3. **Testing asynchronous code:**
If you are testing asynchronous code that expects an error to be thrown, Jest provides a way to handle these scenarios using `async/await` along with `expect.assertions`.

Javascript

function fetchDataAsync() {
       return new Promise((resolve, reject) =&gt; {
           reject(new Error('Async error'));
       });
   }

   test('testing async code with exception', async () =&gt; {
       expect.assertions(1);
       try {
           await fetchDataAsync();
       } catch (error) {
           expect(error).toBeInstanceOf(Error);
       }
   });

Here, `expect.assertions(1)` ensures that the test expects one assertion to be made within the test function.

4. **Handling exceptions in promise rejections:**
If you are dealing with promise rejections and want to test for a specific error type, you can use the `rejects` matcher provided by Jest.

Javascript

function fetchDataPromise() {
       return Promise.reject(new TypeError('Promise rejection'));
   }

   test('testing promise rejection type', () =&gt; {
       return expect(fetchDataPromise()).rejects.toThrow(TypeError);
   });

The `rejects` matcher helps you test promise rejections with specific error types.

By following these steps, you can effectively test the type of a thrown exception in Jest and ensure your code behaves as expected under error conditions. Testing exceptions is a valuable practice in writing robust and reliable code.

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